1. Field of the Invention
The present invention relates to an analog-to-digital converter used in a digital signal processing system, and more specifically to a solution for overcoming saturation of the analog-to-digital converter.
2. Discussion of the Related Art
FIG. 1 schematically shows an exemplary signal processing chain. The shown chain is part of a conventional Discrete Multi-Tone (DMT) demodulator used, for example, in Digital Subscriber Line (DSL) modems.
An analog incoming signal is provided through a hybrid line interface 10 to an analog processing circuit 12. Circuit 12 typically performs an automatic gain control and a low-pass filtering. The analog signal A provided by circuit 12 is converted into digital samples D by an analog-to-digital converter 14. The digital samples D, for example 12-bits wide, are provided to a Digital Signal Processor (DSP) 16 programmed to carry out the required processing. In a DMT demodulator, the DSP will mainly carry out a Fast Fourier Transform (FFT).
FIG. 2 shows an exemplary portion of analog signal A at the input of analog-to-digital converter 14. In a DMT transmission system, the analog signal is the sum of several subcarriers which are modulated in phase and in amplitude, according to random data to be transmitted. As a consequence, the average amplitude of the signal is relatively low. However, from time to time, several subcarriers may be in phase, causing a high amplitude peak, as shown in the middle of the signal. As illustrated on the right of FIG. 2, the amplitude probability density of the signal follows a gaussian curve centered on amplitude zero, i.e. the smaller amplitudes have a higher probability of occurrence than the higher amplitudes.
A problem in designing the analog-to-digital converter 14 is that its range should be adapted to the most probable amplitudes in order to reduce the quantization noise of the conversion to an acceptable value, given the number of bits of the converter.
In practice, the range of the converter, included between values -Vt and +Vt in FIG. 2, is chosen so that the amplitudes of the signal have a given maximum probability of occurring within the range, accepting that, from time to time, there will be amplitudes out of range.
When out-of-range amplitudes occur, the analog-to-digital converter saturates, producing undesirable frequency domain components after the FFT.
In order to avoid the saturation of the converter, it could be devised to extend its range. However, if the range is extended without increasing the number of bits of the converter, the resolution of the converter decreases, causing an increase of the quantization noise.
Thus, the best solution would be to increase the range of the converter and correspondingly increase the number of bits of the converter in order to maintain the resolution. However, increasing the number of bits of the converter increases the complexity of the design of the converter.
Another solution for avoiding saturation of the analog-to-digital converter is to perform a compression of the analog input signal and to perform a corresponding expansion of the digital output signal. This technique is known as "non uniform quantization" and is described in the manual "Digital Communications--Fundamentals and applications", by Bernard Sklar, Prentice-Hall, 1988.